There are conventional image forming apparatus, such as copying machines or laser beam printers (LBP), which form images on sheets.
In FIG. 8, a copying machine as an example of such an image forming apparatus is shown. The copying machine 10 is provided with a document stand 2 composed of a transparent glass plate that is fixed to the top portion of the main body 1 of the copying machine 10. A document pressure cover 3 is configured to press a document D that is being placed at a predetermined position on the document stand 2 with its image surface facing downward, against the document stand 2 to immobilize it. Beneath the document stand 2, there is provided an optical system composed of a lamp 4 for illuminating the document D, reflection mirrors 5, 6 and 7 for directing a reflected light image of the document D to an image processing unit 8 and an imaging lens 11. The lamp 4 and the reflection mirrors 5, 6 and 7 are adapted to be moved in the direction indicated by arrow a at a predetermined speed so as to scan the document D.
An image forming portion 15 includes a photosensitive drum 12, a charger 13 for charging the surface of the photosensitive drum 12 uniformly, a developing device 14 for forming a toner image on the photosensitive drum 12, a transferring charger 19 for transferring the toner image formed on the photosensitive drum 12 onto a sheet S, a stripping charger 20 for stripping the sheet S, on which the toner image has been transferred, from the photosensitive drum 12, and a cleaner 26 for removing toner remaining on the photosensitive drum 12 after the toner image has been transferred. The photosensitive drum 12 is adapted in such a way that an electrostatic latent image is formed, by a light image radiated by a laser unit 9, on the surface of the photosensitive drum 12 that has been charged by the charger 13 and then the electrostatic latent image is developed and transferred onto the sheet S. The sheet S is fed from a sheet cassette 27.
In the downstream of the image forming portion 15, there is provided in sequence, a transport portion 21 for transporting the sheet S on which the toner image has been transferred, a fixing device 22 for fixing the toner image on the sheet S transported by the transport portion 21 to the sheet as a permanent image, and a delivery roller pair 24 for delivering the sheet S on which the toner image has been fixed by the fixing device 22, from the main body 1 of the copying machine. The copying machine has a delivery portion 25 provided externally of its main body 1 for receiving the sheet S delivered by the delivery rollers 24.
There is another type of copying machine such as a copying machine 16 shown in FIG. 9 that has an in-body delivery structure in which sheets are delivered inside its main body in order to save space. The image forming process of this copying machine 16 is substantially the same as that of the copying machine 10 shown in FIG. 8, though a sheet transport path 53 of the transfer portion is configured to guide sheets S in the vertical direction. In addition, a transfer roller 50 is used in place of the transfer charger 19 shown in FIG. 8. In the copying machine 16 shown in FIG. 9, the sheet transport path 53 is provided in the right side portion of the main body 1 of the apparatus. Therefore, a fan 54 is provided on the side of the left side wall of the main body 1 of the apparatus so as to cause air in the main body of the machine to flow from the right to the left of the main body 1 of the apparatus.
There is also another type of copying machine 28 shown in FIG. 10, which is additionally provided with a rotary developing unit 51 and an intermediate transfer belt 52 etc. for forming a color image. This copying machine 28 uses a charging roller 60 in place of the charger 13 shown in FIG. 8.
The fixing device 22 of each of the copying machines 10, 16 and 28 is adapted to nip and transport the sheet while applying heat and pressure to it so as to fix the toner image on the sheet.
In connection with the above descriptions, components of the copying machines 16 and 28 that operate in the same manner as the corresponding components of the copying machine 10 are designated with the same reference signs as the components of the copying machine 10, and the descriptions of those components are omitted.
In the structure like that shown in FIG. 10, in which a fan 55 is provided on the side of the left side wall of the main body 1 of the apparatus to cause air to flow from the right to the left of the main body 1 of the apparatus, there is a risk that the heat generated by the fixing device 22 might be transferred to the rotary developing unit 51 and the intermediate transfer belt 52 etc. The rotary developing unit 51 and the intermediate transfer belt 52 are processing units that use toner, and therefore if their temperatures increase excessively, troubles such as toner blocking or fusion bonding of toner can occur.
In the copying machines 16 and 28 shown in FIGS. 9 and 10, in which sheets are transported vertically along the sheet transport path 53, the distance between the fixing device 22 that fixes toner images on sheets and the delivery portion 25 that delivers the sheets is relatively small as compared to the copying machine 10 shown in FIG. 8, in which sheets are transported horizontally. Therefore, the temperature of the sheets delivered to the delivery portion 25 is high, and so sheets can stick or adhere to each other due to toner images that have not been cooled sufficiently (such phenomena will be referred to as “delivered sheet adhesion” hereinafter).
In addition, the fan 56 of the copying machine 10 shown in FIG. 8 is disposed in the rear side portion of the main body 1 of the apparatus and in the vicinity 61 of the fixing device 22. The fan 54 of the copying machine 16 shown in FIG. 9 is disposed in the vicinity 62 of the left side wall of the main body 1 of the apparatus. The fan 57 of the copying machine 28 shown in FIG. 10 is disposed in the rear side portion of the main body 1 of the apparatus and in the vicinity 63 of the fixing device 22. Therefore, when the fans 56, 54 and 55 are to be replaced due to any trouble, a service person is required to approach to the left side wall or the rear side of the main body 1 of the apparatus for replacement operations. In addition, the above-described arrangements of the fans not only require a long time for maintenance operations but also require the copying machine to be shut down for a long time (i.e. downtime is long).
Furthermore, in the copying machine 16 shown in FIG. 9, in which sheets S are transported vertically along the sheet transport path 53, the leading edges of the sheets may bend to the right or left, and therefore they are often transported under unstable states. Especially, after a sheet has left the fixing device 22 and until it reaches the delivery portion 25, the sheet is apt to be curled due to heat and pressure applied by the fixing device 22, and so it is required to enable stable transportation of the sheets.
Since the fan 57 of the copying machine 28 shown in FIG. 10 is disposed in the rear side portion of the main body 1 of the apparatus and the fixing device 22 is arranged longitudinally in the direction transverse to the direction of the sheet transportation (i.e. the longitudinal direction coincides with the direction perpendicular to the plane of the drawing sheet of FIG. 10), the heat of the fixing device 22 is exhausted at one end of the fixing device 22, and therefore the efficiency of heat exhaust is low.